1. Field of the Invention
The present invention relates to a new photosensitive composition capable of recording a volume hologram, a volume hologram recording medium using the same, and a volume hologram produced using the photosensitive composition.
2. Description of the Related Art
A volume hologram is produced by allowing object lights (object waves) and reference lights (reference waves) each having high coherency (interfering ability) and an equal wavelength to interfere with each other, radiating the resultant lights into a layer made of volume hologram recording material, and recording three-dimensional information about the object, as interference fringes, inside the recording material layer. The interference fringes can be recorded, for example, as a variation in the refractive index (a change in the refractive index) corresponding to bright or dark regions of interference light, inside the recording material layer. Volume holograms are widely used in fields, for example, for design purpose, security purpose, optical element purpose and others since the volume holograms make it possible to represent an object three-dimensionally, have high diffraction efficiency and wave selectivity, and need high-level producing technique.
As photosensitive compositions for producing volume holograms, an OmniDex series available from Du Pont is only one present in the market under mass-production. This material is a dry-developable type photopolymer material which uses a radical polymerizable monomer, a binder resin, a photo-radical polymerization initiator and a sensitizing dye as its major components and make use of a difference in refractive index between the radical polymerizable monomer and the binder resin. That is, when the photosensitive composition formed into a film is subjected to hologram-exposure (exposure for recording holograms), radical polymerization is initiated in regions where the light is intense and a gradient of the concentration of the radical polymerizable monomer is generated accordingly. Thus, the radical polymerizable monomer molecules diffuse and move from regions where the light is weak to the regions where the light is intense. As a result, the radical polymerizable monomers can be made dense or thin dependently on the intensity or weakness of the interfere light, so that a refractive index difference is generated. This material system has the best performance among volume holograms photopolymers which have been reported up to the present time. However, it has been pointed out that the system has problems about heat resistance and transparency. As described above, usually, a hologram-exposure is performed by allowing object waves and reference waves to interfere with each other and be radiated into regions where images are to be recorded. However, it is also possible to perform a hologram-exposure by using of light having no coherency.
A material system using radical polymerization and cationic polymerization together is reported. For example, Japanese Patent No. 2873126 discloses a system using a monomer having a diarylfluorene skeleton as a high refractive index radical polymerizable monomer and a cationic polymerizable monomer which has a lower refractive index than the radical polymerizable monomer. In this system, the high refractive index components are polymerized by radical polymerization at the time of hologram-exposure, and subsequently the images are fixed by cationic polymerization at time of fixation-exposure.
A material system using cationic polymerization is also disclosed in, for example, U.S. Pat. No. 5,759,721. This material system has an advantage of being free from such oxygen inhibition as seen in a radical polymerization system. However, this system has problems that it has low photosensitivity (photospeed) to cationic polymerization and it is difficult to provide this system with sensitivity in a long-wavelength range.
Moreover, Japanese Patent No. 2953200 discloses a material system using a combination of an inorganic material network and a photopolymerizable monomer. When an inorganic material capable of forming the network is used as a binder, there are produced advantages that this material system is superior in heat resistance, environmental resistance and mechanical strength and further the refractive index difference between the photopolymerizable organic monomer and the inorganic material can be made larger. But there are caused problems that the hologram recording film prepared from this material system is somewhat brittle and inferior in properties of flexibility, processability and coating ability and that it is difficult to prepare a homogenous coating material since the compatibility between the inorganic binder and the organic monomer is not good.
Japanese Patent Application National Publication No. 2000-508783 discloses, as a hologram recording material, a material in which metal superfine particles are dispersed in a solid matrix. However, in this material, it is required to provide the matrix with fluidity so that there is a problem of poor solidity.
In particular, known examples of a combination use of a high refractive index binder resin containing an aromatic ring and a low refractive index fluorine-contained monomer include Japanese Patent Application Laid-Open (JP-A) No. 5-210343, 5-210344 and 5-257416. According to this combination, however, the sensitivity at the time of hologram-exposure is not so high since the polymerization reactivity is insufficient. In order to promote the polymerizability of the fluorine-contained monomer, it is stated that a polyfunctional acrylate is added thereto. However, this method obstructs the low refractivity which the fluorine-contained monomer originally has.
A known example of a combination use of a low refractive index binder resin, composed of a fluorine-contained acrylic monomer and a nonfluorine-contained acrylic monomer, and a high refractive index monomer in which an aromatic ring is contained include JP-A No. 6-67588. However, this example also has problems that the low refractivity of the fluorine-contained acrylic monomer is obstruct by copolymerization thereof with the nonfluorine-contained acrylic monomer and further yellowing is liable to be caused by the aromatic ring of the aromatic ring-contained monomer.
Since fluorine-contained compounds generally have a very low refractive index, they are expected as materials for making a refractive index modulation (Δn) large. However, the compounds are poor in compatibility with other blend components such as a binder resin. Thus, the range from which other blend components can be selected is limited and further it is difficult to make the blend ratio of the fluorine-contained compound high. For this reason, in material systems in which a fluorine-contained compound is blended as a refractive index modulating component, the Δn thereof has been hitherto unable to be sufficiently improved. Thus, it has been desired to improve the Δn when such a material is applied to new optical elements.